Field of the Invention
This invention relates to an automobile glass pane, especially a vertically adjustable side pane of laminated glass composed of two thermally toughened single glass sheets, bonded together by a thermoplastic intermediate layer.
Description of the Related Art
An automobile glass pane which can be lowered of this type is known from DE-GM 89 10 916. In this known lateral pane, the outer glass sheet of the laminated glass pane has a thickness of 3 to 4 mm and possesses, as a whole, the usual prestress of a monolithic safety glass pane. The inner glass sheet of the laminated pane has a thickness of 1.5 to 2.5 mm and has no, or very little, prestress. The inner glass sheet also has smaller dimensions than the outer, prestressed glass sheet, and the projecting edge region of the prestressed outer glass sheet serves for guiding the laminated pane in the guide rails of the window frame. The two glass sheets are bonded together by means of a film of thermoplastic 0.8 to 2.0 mm thick.
This known glass pane, as a consequence of its construction, is to a large extent burglar-proof and also has good acoustic insulation properties. The burglar-resistant and penetration-resistant effect is based both upon the relatively thick thermoplastic intermediate layer and upon the feature that the not, or only slightly, prestressed inner glass is not destroyed even if the outer toughened glass sheet is destroyed, due to the thick intermediate film or, in the case of destruction, breaks only into very large fragments so that the window opening in all cases remains closed. This means, however, that even in those cases in which it is necessary to break through the side window panes, namely in the case of an accident, the high resistance of this known lateral glass pane impedes rapid breakage and thereby rapid access to the occupants of the automobile.
In DE-P 40 27 035, which is not admitted to be prior art except as required under 35 U.S.C. 102, each glass sheet in an automobile glass pane of laminated glass has a thickness of 2.0 to 3.0 mm and a prestress lower than the prestress of monolithic safety glass and varying over the area of the glass sheet in such a way that, all around in the edge region, the average tensile stress in the core is 5 to 15% lower than the tensile stress required for breakage behavior in accordance with the standards for the same thickness of pane and that, in the region of the pane area within this edge region, the average tensile stress in the core is 20 to 40% lower than the tensile stress required for breakage behavior in accordance with the standards for the same thickness of pane.
Corresponding to these lower prestress values, which differ from the edge to the central region in these laminated glass panes, the tensile stresses in the core, for a thickness of the glass sheets of 2.0 mm, are 54 to 76 MN/m.sup.2 in the edge region and, with increasing thickness, decrease linearly to 46.7-71.2 MN/m.sup.2 for a sheet thickness of 3 mm, and in the central region, for a glass sheet 2 mm thick, have the values 38 to 64 MN/m.sup.2 and decrease linearly with increasing thickness down to 33-60 MN/m.sup.2 for a sheet thickness of 3.0 mm.
A laminated glass pane having this stress arrangement of its individual glass sheets has the advantageous characteristic that compared with a laminated glass pane of fully prestressed glass sheets, the strength in the middle area of the laminated pane has been reduced to a sufficient extent that, in emergency, the laminated glass pane can be pressed inwards from outside and thus the window opening can be exposed comparatively quickly. Whereas the central region of the two single glass sheets of the laminated glass pane then breaks into very large pieces, and the edge zone disintegrates into comparatively small fragments, which facilitates the removal of the laminated pane from its mounting. The reduction in the strength in the central zone also has a favorable effect, in the case of collision accidents, upon the biomechanical properties of the laminated glass pane, whereas the comparatively higher prestress in the edge zone has the favorable effect that the laminated pane withstands the higher mechanical loadings in the edge region without problems.